I usually buy Newman's Own light balsamic vinaigrette, because it is 45 cals for 2 Tbs. When I use it I measure it out, then throw in a Tbs of enova or olive oil. However, it may be a little too high in sodium or fat for you depending on your goals. When I'm not feeling so lazy I make my own!

Since they're geared for diabetics they generally have about zero sugar or oil in them. I've tried 2, 3, 11, and 19, and they are decent, though they take getting used to. IME, it's the oil in salad dressing that gives it a pleasant texture and taste, so when you use one with no oil, you definitely notice it's absence. However, these taste good, and since you're making them yourself, it's easy to tweak them. Heck, you can even add small amounts of oil until you find it palatable, which you might find to be significantly less than what is put in commercial dressings.

Otherwise, you should check out Whore (I mean 'Whole') Foods or Yuppie Joes (I mean 'Trader Joes') as they have a lot of stuff of this nature. Just be sure to scour the label for badness, as it's remarkable how unhealthy health food can be. As though having a ton of sea salt is somehow better than a ton of sodium...

My local store just started carrying Kraft Light Done Right dressings. Most of them are junk but the Zesty Italian is very good. 25 cals, 2g carbs from plain sugar, 1.5g fat from olive oil and soybean oil. No corn syrup or hydrogenated oil.